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Abstract

We classically measure the entire propagation matrix of a few-mode fiber and use a spatial light modulator to undo modal mixing and recover single-photons launched onto each of the eigenmodes of the fiber at one end, but arriving as mixed modal superpositions at the other. We exploit the orthogonality of these modal channels to improve the isolation between a quantum and classical channel launched onto different spatial and polarization modes at different wavelengths. The spatial diversity of the channels provides an additional 35dB of isolation in addition to that provided by polarization and wavelength.

A schematic of spontaneous four-wave mixing. (a) a pulse enters the photonic crystal waveguide
and is slowed, causing an increase in peak power and interaction time in the waveguide. Two photons
from the pump are annihilated to create signal and idler photons of higher and lower energy, as
described by the arrows in (b). The photons and residual pump then exit the waveguide.

Classically measured (a) amplitude of the propagation matrix U (b) Phase of the
propagation matrix U. (c) Single-photon measurement of UU* and (d)
Numerical evaluation of the amplitude of UU* using (a) and (b).

The extracted heralded single-photon rate (blue squares) did not vary significantly as the power
was increased in the classical channel and was close to the rate when no power is present (blue
dashed line). The measured CAR (black circles) and trend (red line) show values comparable to the
classical channel off (black dotted line). Errors are from Poissonian statistics.